1. Field of the Invention
Aspects of the present invention relate to an organic light emitting display and a method of driving the same, and more particularly, to an organic light emitting display capable of minimizing power consumption and a method of driving the same.
2. Description of the Related Art
Recently, various flat panel displays (FPD) having less weight and volume than cathode ray tubes (CRT) have been developed. The FPDs include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and organic light emitting displays.
Among the FPDs, the organic light emitting displays display images using organic light emitting diodes (OLED) that generate light by the re-combination of electrons and holes. The organic light emitting display has high response speed and is driven with low power consumption.
FIG. 1 is a circuit diagram illustrating a pixel of a conventional organic light emitting display.
Referring to FIG. 1, a pixel 4 of the conventional organic light emitting display includes an organic light emitting diode OLED and a pixel circuit 2 coupled to a data line Dm and a scan line Sn to control the OLED.
The anode electrode of the OLED is coupled to the pixel circuit 2 and the cathode electrode thereof is coupled to a second power source ELVSS. The OLED generates light with predetermined brightness to correspond to current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the OLED to correspond to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. To this end, the pixel circuit 2 includes a second transistor M2 coupled between a first power source ELVDD and the OLED, a first transistor M1 coupled between the second transistor M2, the data line Dm, and the scan line Sn, and a storage capacitor Cst coupled between the gate electrode and the first electrode of the second transistor M2.
The gate electrode of the first transistor M1 is coupled to the scan line Sn, the first electrode thereof is coupled to the data line Dm, and the second electrode thereof is coupled to the storage capacitor Cst. Here, the first electrode is set as one of a source electrode and a drain electrode and the second electrode is set as the other thereof different from the first electrode. For example, when the first electrode is set as the source electrode, the second electrode is set as the drain electrode. The first transistor M1, coupled to the scan line Sn and the data line Dm, is turned on when a scan signal is supplied from the scan line Sn to supply a data signal supplied from the data line Dm to the storage capacitor Cst. At this time, the storage capacitor Cst charges a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, the first electrode thereof is coupled to the other terminal of the storage capacitor Cst and the first power source ELVDD, and the second electrode thereof is coupled to the anode electrode of the OLED. The second transistor M2 controls the amount of current supplied from the first power source ELVDD to the second power source ELVSS via the OLED to correspond to a voltage value stored in the storage capacitor Cst. At this time, the OLED generates light corresponding to the amount of current supplied from the second transistor M2.
In the conventional the pixel 4, the second transistor M2 is driven as a constant current source that supplies uniform current to the OLED in response to a voltage stored in the storage capacitor Cst. Here, in order to operate the second transistor M2 as the constant current source, the second transistor M2 should be driven in a saturation region. Therefore, the voltage between the first power source ELVDD and the second power source ELVSS is set so that the second transistor M2 is driven in the saturation region.
Actually, the voltage between the first power source ELVDD and the second power source ELVSS can be expressed by the following EQUATION 1.ELVDD−ELVSS>Vds—sat+Voled+Vmt+Vmo  [EQUATION 1]
In EQUATION 1, Vds_sat represents the minimum voltage value between the first electrode and the second electrode of the second transistor M2 for driving the second transistor M2 in the saturation region when the maximum current that can be supplied from a pixel circuit 2 to the OLED flows. Voled represents the voltage value applied to the OLED when the maximum current is supplied.
Vmt represents the voltage margin voltage value caused by the process deviation of the second transistor M2 and Vmo represents the margin voltage value corresponding to the process deviation and the temperature characteristic of the OLED. Actually, although the same current is supplied to the OLED, the value of the voltage applied to the OLED changes in response to the temperature at which the OLED is currently driven. Therefore. Vmo is set so that the pixel 4 can be stably driven in consideration of the temperature characteristic of the OLED.
Meanwhile, when the voltage of the first power source ELVDD and the second power source ELVSS is set by the EQUATION 1, power consumption increases. In particular, the margin voltage of Vmo added in consideration of the temperature characteristic occupies 20% to 30% of the power consumption. Therefore, a method of reducing the power consumption by reducing the voltage of Vmo is required.